With the explosive growth of the World Wide Web, the race to deliver high-bandwidth access to the consumer is on. Competing bandwidth-delivery schemes include conventional analog modems, “digital” modems, integrated services digital networks (ISDN), asymmetrical digital subscriber line (ADSL) and its siblings, referred to collectively as xDSL, cable modems, and satellite network links.
For practical and historical reasons, companies offering data network access (e.g., network service providers, or “NSPs”) today typically rely on at least some portion of the public system telephone network (PSTN) to deliver their product. Analog modems, digital modems, ISDN, and xDSL all utilize the consumer's “local loop”, i.e., the copper wire twisted pairs(s) connecting the consumer's residence or business with a central office or branch. An analog modem connection may also connect to the ISP through a local analog loop. The fastest analog modems utilize a “half-digital” connection, i.e., the ISP connects to the PSTN digitally while each of their customers utilizes an analog connection. And with ISDN and xDSL, the connection utilizing the local loop is all digital.
A typical NSP may provide simultaneous data network access to hundreds or even thousands of customers through a single gateway connected to the PSTN. Such a gateway usually will not comprise a bank of discrete modems each connected to a phone line, but rather a group of modem cards, each containing multiple processors, all connected to a high-speed data bus. The high-speed data bus connects to the PSTN through an access concentrator. The access concentrator typically transmits and receives signals to and from the PSTN over multiple T1 or E1 connections, and performs format conversion between the T1/E1 format and the high-speed data bus format.
Because the quality of local loop connections can vary widely, sophisticated modems “train” to a best possible connect speed for each connection by a process of line probing. For instance, a “half-digital” modem connection utilizing the V.90 specification tests line transmission characteristics for each of the 256 possible 8-bit digital symbols during training. Training allows the modem pair to determine a subset of these symbols that can be received reliably by the analog end of that communication channel. The training process typically takes at least several seconds for each connection, and the results of the process are used to configure an internal state for each modem in the modem pair.